Field of the Invention
The present invention describes a process for separating mixtures of m- and p-dichlorobenzenes, p-dichlorobenzene being separated from such mixtures and an enriched m-dichlorobenzene being removed. The separation is carried out in the liquid phase on pentasil zeolites in the liquid phase with the use of solvents.
2. Description of the Related Art
In the chlorination of benzene or monochlorobenzene to dichlorobenzene, a mixture of the three isomeric dichlorobenzenes is generally formed. The fraction containing the dichlorobenzenes can easily be separated on account of the different boiling and melting points from the other products in the chlorination mixture, for example the starting substances and higher chlorination products. The separation of the individual dichlorobenzene isomers in this fraction is however difficult and extremely complicated. The dichlorobenzene fraction freed from the by-products is first of all distilled, m- and p-dichlorobenzene being obtained as top product and o-dichlorobenzene being obtained as bottom product. Since m- and p-dichlorobenzene have identical boiling points, the further separation is carried out by fractional crystallisation, a large part of the p-dichlorobenzene being obtained in pure form and an m-dichlorobenzene concentrate remaining behind, which, depending on the degree of crystallisation, contains amounts of from 70 to 80% by weight of m-dichlorobenzene and from 30 to 20% by weight of p-dichlorobenzene (EP 012,891). Also, adsorptive processes for the separation or further separation of mixtures of dichlorobenzenes using molecular sieves are in principle known. For example, EP 334,025 describes the separation of halogenated aromatics on various zeolites in the presence of aromatic hydrocarbons. The separation of m- and p-dichlorobenzene on pentasil zeolites is, however, not disclosed in this description. EP 278,680, JP 62/175,433 (1987) and U.S. Pat. No. 4,996,380 describe the separation of m-dichlorobenzene from mixtures thereof with o- and p-dichlorobenzene without using eluants. These processes are suitable for separating small amounts of m-dichlorobenzene from mixtures containing mainly o- and p-dichlorobenzene. Since, however, an m-dichlorobenzene concentrate is easily obtained in the working-up of technical dichlorobenzene product mixtures by distillation and a simple crystallisation step, from which the contained residues of p-dichlorobenzene can be separated only by time-consuming and energy-consuming distillation and crystallisation steps, a process that permits an economical separation of the p-dichlorobenzene from an m-dichlorobenzene concentrate is extremely desirable.
Such a separation of small amounts of p-dichlorobenzene from mixtures containing mainly m-dichlorobenzene using molecular sieves has not been satisfactorily solved according to the prior art.
U.S. Pat. No. 4,571,441 and JP 58/131924 (1983) describe the separation of p-dichlorobenzene from mixtures containing o- and m-dichlorobenzene using faujasite X and faujasite Y, each of which contain silver, copper, sodium or potassium or a plurality thereof as exchangeable cations. Substituted aromatic hydrocarbons are used as desorbents. The selectivities achieved in these processes are extremely low and do not guarantee an economical process.